If you can get some very professional, light and small size equipment for the Camino, use a 30 (woman) or 40 (man) liter backpack. But if you have ordinary stuff, 40 l (woman) or 50 l (man) will fit you much better. Avoid buying an oversized backpack, because you won't resist packing it full...

Bear it in mind that the ideal weight of your backpack is about 10% of your bodyweight.